Try a Magnum.

It sounds to me like his cock is tapered...thick at the bottom and not as thick at the top, if there was slippage at the top.

Mine is tapered too and Magnum XL's work fine, they slip a bit but only round the top and its probably due to my foreskin moving up and down...

But yeah try Magnums. Or maybe Condomi XXL. They aren't as wide as Magnum XL's but are close to 8" long.